Gee whiz! A fine case of vorticity

Tornado Alley - Air Apparent
October 29, 1999

Polite conversation? Alan Thorpe finds deeper interest in weather.

Meteorology is a fascinating discipline - on the one hand academic and on the other extremely practical. This is what attracts many physics, mathematics and chemistry graduates to the subject. The balance between the gee whiz of spectacular events such as tornadoes and the basic scientific understanding of the fluid dynamics of the atmosphere is a thread common to both these books.

Their publication is timely. Mark Monmonier's book on weather maps, Air Apparent , coincided with the Met Office's last-ever hand-drawn North Atlantic weather chart, produced on March 29 1999. The first such chart was produced for May 23 1867 and a complete set of charts is housed in the National Meteorological Archive.

One might ask why the computer did not deal a fatal blow to the human analyst a lot sooner. The answer lies, in part, in the complexity of the conceptual model of the atmospheric features such as fronts marked on the charts and the consequent difficulty of automating the plotting process.

Air Apparent includes the history of the weather map. Monmonier is a professor of geography who specialises in cartography. The weather map synthesises a multitude of observational and forecast data into a snapshot of the atmosphere. The real benefit of these maps is proved by the fact that much the same style has been employed since the advent of the first map in 1816, arguably made by Heinrich Brandes, a German professor of physics, showing weather conditions for the year 1783.

Most people encounter weather charts in their newspaper or on television as a way of expressing a weather forecast. Admiral Fitzroy, of Beagle fame,was responsible for the first general weather forecast by the Met Office on August 1 1861. The reaction of the scientific establishment was adverse: it regarded these "highly empirical weather predictions (as) unsound and premature". This deepened Fitzroy's already depressed state, leading to his suicide on April 30 1865. When the Met Office was transferred to the Royal Society in 1866, the storm forecasts were suspended and were not resumed until 1879.

As a professional meteorologist I am fascinated by Monmonier's historical survey of the development of the weather map. Other parts of the book describe, in a less insightful or authoritative way, the development of satellites and radar as tools with which to view the weather. An opportunity has been missed here: to look into the future of virtual-reality descriptions of evolving weather systems. In common with many fields of the physical sciences, meteorology will benefit from new ways to visualise complex four-dimensional datasets. Nevertheless, I recommend Air Apparent to meteorologists and cartographers.

It was a curious feeling to be reading the newspaper headline "400 ft twister wreaks havoc as Britain is battered by freak weather" while travelling home from work by train via Wokingham with Howard Bluestein's Tornado Alley in my briefcase. For Wokingham was the town where, on July 9 1959, there was a hailstorm which was one of the first to be studied by radar. Subsequent analysis of the data led to the first picture of the three-dimensional flow characteristics of what are now known as supercell storms, from which most tornadoes arise. Most people are shocked to learn that the UK has tens of tornadoes each year; it is also surprising that the humble Wokingham hailstorm enjoys such fame in the tornado-studying community.

Bluestein is a well-known and respected professor of meteorology from the heart of tornado alley in Oklahoma. His new book is a tour de force with fantastic photographs of tornadoes and storm clouds from his own collection intermingled with an accessible description of tornado science.

Much of the book is an extended diary of many years of storm and tornado chasing. The combination of illustrations worthy of a coffee-table book with detailed discussion of the fluid dynamics of tornado formation and structure works really well and the book should have a wide readership. It is a fine example of how the public understanding of science can be achieved without any "dumbing down".

Some historical nuggets are most intriguing. The word tornado was banned from weather forecasts by the US government in 1887 because it was believed "that the harm done by such a prediction would be greater than that which results from the tornado itself". The ban was not totally lifted until 1938.

These days forecasts of tornadoes are based on being able to predict a probability of occurrence of the thunderstorms that spawn smaller-scale embedded tornadoes. This is feasible up to a day or two in advance. But the precise location and timing of the tornado still relies on a visual sighting combined with radar detection, providing only a few, or at most a few tens of, minutes' warning.

Although the fluid dynamics of tornadoes obey classical physical laws, they are nonetheless exceedingly complex and the prediction of tornado occurrence remains beyond the capability of the current generation of weather-prediction models. The evolution of the parent thunderstorm and its tornado offspring involve all the mechanisms for generation of spin (vorticity) about a vertical axis: the stretching of existing vertical vorticity, the tilting of existing horizontal vorticity as air rises up and down in the storm, and the so-called baroclinic generation from the strong horizontal contrast of temperature between the warm ascending column and the cold descending column. How these elements interact to create a tornado is still something of a mystery, despite numerical simulations of the storm system that exhibit large-scale generation of vorticity.

Bluestein refers to the large band of, sometimes fanatical, amateur stormchasers. This committed group have their own jargon and, for example, would not be seen dead uttering the word twister in favour of "tornado". The amateurs have clearly contributed to the professional's knowledge of these phenomena. The professionals are not always as clever as the amateurs - on a recent research flight to observe a tornado with airborne Doppler radar, scientists received nasty injuries because many of them were not wearing seat belts and so during a particularly bumpy ride hit the aircraft ceiling!

There have been big advances in the observations of tornadoes in recent years. Doppler radar mounted on a small truck - "Doppler on Wheels" - is the prime tool, but the latest technique is Eldora dual-Doppler radar mounted on a meteorological research aircraft. This gives a full three-dimensional wind-field description. The maximum wind speed in a tornado is very difficult to measure but is now thought to be in excess of 300 miles per hour.

Bluestein has produced a gem of a book and Oxford University Press is to be congratulated on the quality of production: the photographs are superb and the layout perfect.

Alan J. Thorpe is director of climate research, The Meteorological Office.

Tornado Alley: Monster Storms of the Great Plains

Author - Howard B. Bluestein
ISBN - 0 19 510552 4
Publisher - Oxford University Press
Price - £17.99
Pages - 180

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